1) The document discusses semi-solids like ointments, creams, pastes and gels. It describes the three layers of skin - epidermis, dermis and hypodermis - and factors that influence skin penetration like vehicle, pH, drug properties, skin conditions and more.
2) It also discusses mechanisms of drug permeation through skin and various penetration enhancement techniques using chemicals or physical methods. Common chemical penetration enhancers mentioned include surfactants, fatty acids, alcohols, dimethyl sulfoxide and others.
3) The mechanisms of different penetration enhancers are explained, such as their effects on skin lipids and proteins. Ideal properties of penetration enhancers and their